Dopant-additive synergism enhances perovskite solar modules.

Perovskite solar cells (PSCs) are among the most promising photovoltaic technologies owing to their exceptional optoelectronic properties1,2. However, the lower efficiency, poor stability and reproducibility issues of large-area PSCs compared with laboratory-scale PSCs are notable drawbacks that hinder their commercialization3. Here we report a synergistic dopant-additive combination strategy using methylammonium chloride (MACl) as the dopant and a Lewis-basic ionic-liquid additive, 1,3-bis(cyanomethyl)imidazolium chloride ([Bcmim]Cl). This strategy effectively inhibits the degradation of the perovskite precursor solution (PPS), suppresses the aggregation of MACl and results in phase-homogeneous and stable perovskite films with high crystallinity and fewer defects. This approach enabled the fabrication of perovskite solar modules (PSMs) that achieved a certified efficiency of 23.30% and ultimately stabilized at 22.97% over a 27.22-cm2 aperture area, marking the highest certified PSM performance. Furthermore, the PSMs showed long-term operational stability, maintaining 94.66% of the initial efficiency after 1,000 h under continuous one-sun illumination at room temperature. The interaction between [Bcmim]Cl and MACl was extensively studied to unravel the mechanism leading to an enhancement of device properties. Our approach holds substantial promise for bridging the benchtop-to-rooftop gap and advancing the production and commercialization of large-area perovskite photovoltaics.

Retarding solid-state reactions enable efficient and stable all-inorganic perovskite solar cells and modules.

All-inorganic CsPbI3 perovskite solar cells (PSCs) with efficiencies exceeding 20% are ideal candidates for application in large-scale tandem solar cells. However, there are still two major obstacles hindering their scale-up: (i) the inhomogeneous solid-state synthesis process and (ii) the inferior stability of the photoactive CsPbI3 black phase. Here, we have used a thermally stable ionic liquid, bis(triphenylphosphine)iminium bis(trifluoromethylsulfonyl)imide ([PPN][TFSI]), to retard the high-temperature solid-state reaction between Cs4PbI6 and DMAPbI3 [dimethylammonium (DMA)], which enables the preparation of high-quality and large-area CsPbI3 films in the air. Because of the strong Pb-O contacts, [PPN][TFSI] increases the formation energy of superficial vacancies and prevents the undesired phase degradation of CsPbI3. The resulting PSCs attained a power conversion efficiency (PCE) of 20.64% (certified 19.69%) with long-term operational stability over 1000 hours. A record efficiency of 16.89% for an all-inorganic perovskite solar module was achieved, with an active area of 28.17 cm2.

Revealing the Enhanced Thermoelectric Properties of Controllably Doped Donor-Acceptor Copolymer: The Impact of Regioregularity.

Albeit considerable attention to the fast-developing organic thermoelectric (OTE) materials due to their flexibility and non-toxic features, it is still challenging to design an OTE polymer with superior thermoelectric properties. In this work, two "isomorphic" donor-acceptor (D-A) conjugated polymers are studied as the semiconductor in OTE devices, revealing for the first time the internal mechanism of regioregularity on thermoelectric performances in D-A type polymers. A higher molecular structure regularity can lead to higher crystalline order and mobility, higher doping efficiency, order of energy state, and thermoelectric (TE) performance. As a result, the regioregular P2F exhibits a maximum power factor (PF) of up to 113.27 µW m-1  K-2 , more than three times that of the regiorandom PRF (35.35 µW m-1  K-2 ). However, the regular backbone also implies lower miscibility with a dopant, negatively affecting TE performance. Therefore, the trade-off between doping efficiency and miscibility plays a vital role in OTE materials, and this work sheds light on the molecular design strategy of OTE polymers with state-of-the-art performances.

Emission Spectroscopy Investigation of the Enhancement of Carrier Collection Efficiency in AgBiS2-Nanocrystal/ZnO-Nanowire Heterojunction Solar Cells.

Eco-friendly solar cells were fabricated using interdigitated layers comprising ZnO nanowires (NWs) and infrared absorbing AgBiS2 nanocrystals (ITO/ZnO NWs/AgBiS2/P3HT/Au). The quality of ZnO NWs was studied using photoluminescence and Raman spectroscopy to identify the defects in ZnO NWs influencing solar cell performance. Oxygen vacancies and Zn interstitial sites, among various recombination sites, were observed to be the main sites for carrier recombination, which hinders the carrier collection in the solar cells. Accordingly, the power conversion efficiency of AgBiS2 solar cells exhibited a good correlation with the number of oxygen vacancies. The structural order and electron-phonon interaction in ZnO NWs were also investigated via Raman scattering spectroscopy. A lower concentration of oxygen vacancies and zinc interstitials (Zni) resulted in a higher structural order as well as a weaker electron-phonon interaction in ZnO NWs. When ZnO NWs were treated at 500 °C in oxygen with the lowest oxygen vacancy concentration, the solar cells (500-O2 solar cell (SC)) demonstrated an external quantum efficiency of approximately 70% in the visible region and a corresponding internal quantum efficiency of more than 80%. The 500-O2 SC exhibited a power conversion efficiency of 5.41% (JSC = 22.21 mA/cm2, VOC = 0.41 V, and FF = 60%) under quasi one-sun illumination. New methods that can efficiently reduce oxygen vacancies and Zni without affecting the structural order of ZnO NWs would further enhance the carrier collection efficiency. Moreover, since ZnO is a key electron transport material for constructing not only colloidal quantum dot solar cells but also other emerging solar cells, such as organic thin-film solar cells, the present findings provide significant information for improving their performance.

Dopant-Free Hole Transport Materials Afford Efficient and Stable Inorganic Perovskite Solar Cells and Modules.

The emerging CsPbI3 perovskites are highly efficient and thermally stable materials for wide-band gap perovskite solar cells (PSCs), but the doped hole transport materials (HTMs) accelerate the undesirable phase transition of CsPbI3 in ambient. Herein, a dopant-free D-π-A type HTM named CI-TTIN-2F has been developed which overcomes this problem. The suitable optoelectronic properties and energy-level alignment endow CI-TTIN-2F with excellent charge collection properties. Moreover, CI-TTIN-2F provides multisite defect-healing effects on the defective sites of CsPbI3 surface. Inorganic CsPbI3 PSCs with CI-TTIN-2F HTM feature high efficiencies up to 15.9 %, along with 86 % efficiency retention after 1000 h under ambient conditions. Inorganic perovskite solar modules were also fabricated that exhibiting an efficiency of 11.0 % with a record area of 27 cm2 . This work confirms that using efficient dopant-free HTMs is an attractive strategy to stabilize inorganic PSCs for their future scale-up.

Visualization of halide perovskite crystal growth processes by in situ heating WAXS measurements.

We observed the crystallization dynamics of halide perovskite crystals (CH3NH3PbI3) by in situ heating wide-angle X-ray scattering measurements. As a result, we revealed that crystal growth occurs during the conversion of complexes to perovskite crystals.

Eco-Friendly AgBiS2 Nanocrystal/ZnO Nanowire Heterojunction Solar Cells with Enhanced Carrier Collection Efficiency.

AgBiS2 nanocrystals (NCs) are nontoxic, lead-free, and near-infrared absorbing materials. Eco-friendly solar cells were constructed using interdigitated layers of ZnO nanowires (NWs) and AgBiS2 NCs, with the aim of elongating the otherwise short carrier diffusion length of the AgBiS2 NC assembly. AgBiS2 NCs were uniformly infiltrated into the ZnO NW layers using a low-cost and easily scalable dip coating method. The resulting ZnO NW/AgBiS2 NC interdigitated structures provided efficient carrier pathways in constructed nanowire solar cells (NWSCs), composed of a transparent electrode/ZnO NW/AgBiS2 NC interdigitated layer/P3HT hole transport layer/Au. The photocurrent external quantum efficiency (EQE) in the visible to near-infrared regions was enhanced compared to those of the control solar cells made with ZnO/AgBiS2 tandem layered structures. The maximum EQE for the NWSCs reached 82% in the visible region, which is higher than the EQE values previously reported for solar cells fabricated with ZnO/AgBiS2 NCs. Air stability tests on unsealed NWSCs demonstrated that 90% or more of the initial power conversion efficiency was maintained even after 6 months.

Control of Molecular Orientation of Spiro-OMeTAD on Substrates.

2,2',7,7'-Tetrakis(N,N-di-p-methoxyphenylamino)-9,9'-spirobifluorene (spiro-OMeTAD) is utilized as a p-type semiconductor layer in perovskite solar cells and solid-state dye-sensitized solar cells. Spiro-OMeTAD has been known to have a spiro center, leading to a random orientation. Although the molecular orientation of organic semiconductor materials influences the conductivity, which is directly related to semiconductor device characteristics, the molecular orientation of spiro-OMeTAD has not been fully discussed. In this study, we prepared spiro-OMeTAD layers on various substrates and investigated their orientation by grazing-incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine structure (NEXAFS). Additionally, we demonstrated that the molecular orientation of spiro-OMeTAD could be controlled by changing their surface energies by changing the substrate materials. Consequently, we could improve the electrical conductivity by improving its molecular orientation. The results of this study provide a guideline for the preparation of organic semiconductor material layers using the wet-coating method.

Carbazole-Terminated Isomeric Hole-Transporting Materials for Perovskite Solar Cells.

A set of novel hole-transporting materials (HTMs) based on π-extension through carbazole units was designed and synthesized via a facile synthetic procedure. The impact of isomeric structural linking on their optical, thermal, electrophysical, and photovoltaic properties was thoroughly investigated by combining the experimental and simulation methods. Ionization energies of HTMs were measured and found to be suitable for a triple-cation perovskite active layer ensuring efficient hole injection. New materials were successfully applied in perovskite solar cells, which yielded a promising efficiency of up to almost 18% under standard 100 mW cm-2 global AM1.5G illumination and showed a better stability tendency outperforming that of 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene. This work provides guidance for the molecular design strategy of effective hole-conducting materials for perovskite photovoltaics and similar electronic devices.

Crystal Systems and Lattice Parameters of CH3NH3Pb(I1-xBrx)3 Determined Using Single Crystals: Validity of Vegard's Law.

Metal halide perovskites are promising materials for light absorbers in solar cell applications. Use of the Br/I system enables us to control band gap energy and improves the efficiency of solar cells. Precise knowledge of lattice parameters and band gap energies as functions of compositions are crucially important for developing the devices using those materials. In this study, we have determined lattice parameters and band gap energies of CH3NH3Pb(I1-xBrx)3, one of the most intensively studied mix-halide perovskites, as functions of Br content x. We measured accurate Br contents and lattice parameters of CH3NH3Pb(I1-xBrx)3 (0 ≤ x ≤ 1) using single-crystalline samples by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) measurements, respectively. The CH3NH3Pb(I1-xBrx)3 crystal system is tetragonal for x ≤ 0.06 and cubic for x ≥ 0.08 at 300 K. Lattice parameters of CH3NH3Pb(I1-xBrx)3 strictly follow Vegard's law; i.e., they are linearly dependent on x. We give linear expressions of x of lattice parameters for the tetragonal and cubic phases of CH3NH3Pb(I1-xBrx)3 at 300 K. We have shown that these expressions can be used for determining the Br contents of CH3NH3Pb(I1-xBrx)3 polycrystalline thin-film samples based on XRD measurements and, in addition, demonstrated that XPS measurements on polycrystalline samples may be erroneous because of impure ingredients in the samples. Furthermore, we determined band gap energies of CH3NH3Pb(I1-xBrx)3 (0 ≤ x ≤ 1) at room temperature using absorption spectra of polycrystalline thin films taking account of excitonic effects.

Low-Temperature Synthesized Nb-Doped TiO2 Electron Transport Layer Enabling High-Efficiency Perovskite Solar Cells by Band Alignment Tuning.

An Nb-doped TiO2 (Nb-TiO2) film comprising a double structure stacked with a bottom compact layer and top mesoporous layers was synthesized by treating a Ti precursor-coated substrate using a one-step low-temperature steam-annealing (SA) method. The SA-based Nb-TiO2 films possess high crystallinity and conductivity, and that allows better control over the conduction band (CB) of TiO2 for the electron transport layer (ETL) of the perovskite solar cells by the Nb doping level. Optimization of power conversion efficiency (PCE) for the Nb-TiO2-based ETL was combined with the CB level tuning of the mixed-halide perovskite by changing the Br/I ratio. This band offset management enabled to establish the most suitable energy levels between the ETL and the perovskites. This method was applied to reduce the band gap of perovskites to enhance the photocurrent density while maintaining a high open-circuit voltage. As a result, the optimal combination of 5 mol % Nb-TiO2 ETL and 10 mol % Br in the mixed-halide perovskite exhibited high photovoltaic performance for low-temperature device fabrication, achieving a high-yield PCE of 21.3%.

Effect of Silicon Surface for Perovskite/Silicon Tandem Solar Cells: Flat or Textured?

Perovskite and textured silicon solar cells were integrated into a tandem solar cell through a stacking method. To consider the effective structure of silicon solar cells for perovskite/silicon tandem solar cells, the optic and photovoltaic properties of textured and flat silicon surfaces were compared using mechanical-stacking-tandem of two- and four-terminal structures by perovskite layers on crystal silicon wafers. The reflectance of the texture silicon surface in the range of 750-1050 nm could be reduced more than that of the flat silicon surface (from 2.7 to 0.8%), which resulted in increases in average incident photon to current conversion efficiency values (from 83.0 to 88.0%) and current density (from 13.7 to 14.8 mA/cm2). Using the texture surface of silicon heterojunction (SHJ) solar cells, the significant conversion efficiency of 21.4% was achieved by four-terminal device, which was an increase of 2.4% from that of SHJ solar cells alone.

Lead-free perovskite solar cells using Sb and Bi-based A3B2X9 and A3BX6 crystals with normal and inverse cell structures.

Research of CH3NH3PbI3 perovskite solar cells had significant attention as the candidate of new future energy. Due to the toxicity, however, lead (Pb) free photon harvesting layer should be discovered to replace the present CH3NH3PbI3 perovskite. In place of lead, we have tried antimony (Sb) and bismuth (Bi) with organic and metal monovalent cations (CH3NH3+, Ag+ and Cu+). Therefore, in this work, lead-free photo-absorber layers of (CH3NH3)3Bi2I9, (CH3NH3)3Sb2I9, (CH3NH3)3SbBiI9, Ag3BiI6, Ag3BiI3(SCN)3 and Cu3BiI6 were processed by solution deposition way to be solar cells. About the structure of solar cells, we have compared the normal (n-i-p: TiO2-perovskite-spiro OMeTAD) and inverted (p-i-n: NiO-perovskite-PCBM) structures. The normal (n-i-p)-structured solar cells performed better conversion efficiencies, basically. But, these environmental friendly photon absorber layers showed the uneven surface morphology with a particular grow pattern depend on the substrate (TiO2 or NiO). We have considered that the unevenness of surface morphology can deteriorate the photovoltaic performance and can hinder future prospect of these lead-free photon harvesting layers. However, we found new interesting finding about the progress of devices by the interface of NiO/Sb3+ and TiO2/Cu3BiI6, which should be addressed in the future study.

All-inorganic inverse perovskite solar cells using zinc oxide nanocolloids on spin coated perovskite layer.

We confirmed the influence of ZnO nanoparticle size and residual water on performance of all inorganic perovskite solar cells. By decreasing the size of the ZnO nanoparticles, the short-circuit current density (Jsc) and open circuit photovoltage (Voc) values are increased and the conversion efficiency is improved. Although the Voc value is not affected by the influence of residual water in the solution for preparing the ZnO layer, the Jsc value drops greatly. As a result, it was found that it is important to use the oxide nanoparticles with a small particle diameter and to reduce the water content in the oxide forming material in order to manufacture a highly efficient all inorganic perovskite solar cells.

Synthesis of organic photosensitizers containing dithienogermole and thiadiazolo[3,4-c]pyridine units for dye-sensitized solar cells.

Dithienogermole (DTG) is a germanium-bridged bithiophene system that has been applied as a building unit of conjugated materials for organic electronic devices, including organic photovoltaics and organic light emitting diodes. However, DTG has not been used as a component of sensitizers for dye-sensitized solar cells (DSSCs). In this work, we have synthesized three D-π-A-π-A type sensitizers containing DTG and thiadiazolo[3,4-c]pyridine (PTz). We expected that combining DTG and a strong acceptor PTz would give rise to a strong absorption in the visible region. In addition, we introduced bulky 2-ethylhexyl groups on the germanium atom to prevent dye aggregation on TiO2 films. Three DTG-containing dyes with different anchor units were synthesized and their optical/electrochemical properties were investigated. The DTG-containing dyes exhibited broad and strong absorption bands around 600 nm on TiO2. We fabricated DSSCs based on the DTG-containing dyes. The onsets of incident photon to current conversion efficiency (IPCE) spectra reached 900 nm and a maximal power conversion efficiency of 2.76% was achieved.

Novel near-infrared carboxylated 1,3-indandione sensitizers for highly efficient flexible dye-sensitized solar cells.

Three novel metal-free organic dyes (DN458, DN475 and DN484) were designed for use in plastic-substrate dye-sensitized solar cells (PDSCs). The photoelectric conversion region of DN475 was successfully expanded into the near-infrared region. As a result, an energy conversion efficiency of 5.76% was achieved.

A new cosensitization method using the Lewis acid sites of a TiO2 photoelectrode for dye-sensitized solar cells.

Co-sensitized dye-sensitized solar cells using black dye and a pyridine-anchor dye (NI5 or YNI-2) showing site-selective adsorption behaviour at the TiO2 surface have been prepared for the first time to reduce the competitive adsorption between the two dyes.